Known projection-type systems, such as a projector, include the invention described in Japanese Patent Laid-Open Publication No. 2002-49097. This invention is an application of the invention described in U.S. Pat. No. 5,608,294 (Japanese Patent Announced Publication No. HEI 10-501919), i.e., a known method of a.c. lighting a high-pressure discharge lamp such that “arc flicker can be suppressed by superimposing a pulse current (which is a pulse current of the same polarity as the lamp current and generated for a duration that is reduced from a half cycle of the lamp current by a factor of a predetermined number) on a portion of the latter half of each half cycle of a.c. lamp current (C). The invention is to provide an optimal projection-type system having a reduced possibility of gradation disorder by superimposing such a pulse current on the lamp current in synchronism with a specific color segment of the color filter incorporated in this projection-type system. The “gradation” means the level (degree) of lightness varying on a digital basis in such a manner as white→gray→black. If disorder of gradation occurs, the lightness of each color in an image projected on a screen also falls into disorder.
FIG. 3 is a diagram illustrating the waveform of a.c. lamp current (C) and the waveform of light (Φ) applied to a color filter according to the aforementioned conventional art, and FIG. 5 is a view illustrating electrodes and a state of arc in the a.c. lamp (C). Projection-type systems of this kind involve the following problems caused by the use of alternate current as lamp current (C). That is, (i) the luminescent spot of arc (A′) appears at two points (K1′) and (K2′) because the direction of emission of electrons between electrodes (1a′) and (1b′) in a high-pressure discharge lamp changes as the a.c. lamp current (C) alternates between a positive half cycle (c1) and a negative half cycle (c2). Specifically, the luminescent spot (K1′)(K2′) of arc (A′) appears not at the midpoint between the electrodes (1a′) and (1b′) but at a point closer to one electrode (1a′) or (1b′). Accordingly, in the case of a.c. lighting, luminescent spots (K1′) and (K2′) are produced alternately as current (C) alternates. That is, the luminescent spot (K1′) appears during positive half cycle (C1) of current (C), while the luminescent spot (K2′) appears during negative half cycle (C1) of current (C).
Since the two electrodes (1a′) and (1b′) are not completely identical with each other, the condition of electrons emitted from alternate one of the electrodes (1a′) and (1b′) varies as the direction of current (C) flowing across the electrodes (1a′) and (1b′) alternates. This means that the condition of light emitted from the high-pressure discharge lamp varies as the half cycles (c1) and (c2) alternate with each other. Stated otherwise, light (Φ) produced by emission of electrons and applied to the color filter during positive half cycle (c1) and light (Φ) applied to the color filter during negative half cycle (c2) are not exactly the same, and an error ΔΦ takes place therebetween. Such an error causes gradation disorder in a projection-type system employing such an optical element (4) as a DMD (digital mirror device).
(ii) Moreover, since the polarity alternates after superimposition of pulse current according to the aforementioned conventional art as described above, a variation in luminous flux with varying current (C) describes a complicated waveform having an underchute (U). Since it is required that such light (Φ) varying complicatedly be applied to a desired color segment of the color filter incorporated in the projection-type system, the DMD need be controlled in order to obtain a required color balance by correcting and controlling the variation in luminous flux.
However, a variation in the waveform of luminous flux corresponding to a portion of the lamp current on which the pulse current is superimposed is very complicated and is difficult to reproduce. For this reason, even under the aforementioned control for correction, the projection-type system employing such a DMD cannot completely resolve the gradation disorder. This is the cause of a gradation disorder specific to the use of an a.c. lamp. Thus, the pulse waveform obtained during positive half cycle (c1) is not exactly the same as that obtained during negative half cycle (c2). The influence from this phenomenon becomes particularly great when the pulse current is superimposed on a.c. lighting current (C) in synchronism with a color segment other than the white segment, for example, a red color segment.
(iii) In addition, in the case of the conventional method of superimposing pulse current (P) on a.c. lamp current (C), the position where pulse current (P) is to be superimposed for preventing the occurrence of flicker is limited to a portion of the latter half of each of half cycles (c1) and (c2). Stated otherwise, it is impossible to superimpose pulse current (P) at a desired position and, hence, it is difficult to superimpose pulse current (P) synchronously with plural segments (3a) . . . of the color filter. If such superimposition is done forcibly, the waveform obtained during positive half cycle (c1) differs from that obtained during negative half cycle (c2) and a d.c. component is produced in a.c. lamp current (C), which is considered to affect the life characteristic of the high-pressure discharge lamp undesirably.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2002-49097
Patent Document 2: U.S. Pat. No. 5,608,294 (Japanese Patent Announced Publication No. HEI 10-501919)